Mutants blocked or altered at different steps in the synthesis, assembly, or secretion of Ig by mouse myeloma cells have been isolated and are being characterized. Currently the factors important for Ig secretion are being investigated using two classes of mutants: those exhibiting altered protein structure and those aberrant in secretion without any apparent alteration in protein structure. As an example of the first class, we are studying mutants of the IgG3, lambda-cell line Y5606, which synthesize heavy chains of slightly altered molecular weight that assemble into HL half molecules that are not secreted. We will sequence these altered heavy chains by making cDNAs from them. As an example of the second class, we are studying a mutant of the IgA, lambda-cell line J558, which synthesizes apparently normal H and L chains that assemble normally but are not secreted. Somatic cell hybrids of this cell line now secrete Ig, suggesting that the defect is in the myeloma cell, not in the primary structure of the Ig. We have established methods for transfecting Ig genes into lymphoid cells and having them expressed. We are now using these methods to explore Ig gene and protein function. We have prepared a transfection vector that contains the entire MPC-11 gamma2b gene. Experiments have shown that, when this gene is transfected into lymphoma cells, the predominant mRNA made is of the membrane type; when we transfect this gene into myeloma cell, the predominant mRNA is of the secreted form. We are now using this gene to analyze the DNA sequences necessary for the expression of membrane versus secreted Ig mRNA. (AB)